Pipe Jacking

Construction proceeds in a dig–push–add cycle. The crew excavates as far ahead of the pipe as the ground will stand unsupported (0.5–1.5 m for most soils), the pipe is pushed into the void, and when the pipe is fully in, a new section is welded behind it and the cycle starts again. How far to advance in one go is decided by the ground's stand-up time: stiff clay holds itself while the face is open, flowing sand or ground below the water table does not.

The method's clearest limit is diameter. Because a person has to work inside, it cannot in practice be used for pipes under 1200 mm. Yet the same feature gives its greatest advantage: it is the only method with direct access to the tunnel face. A large boulder, an old foundation or an abandoned line on the route is seen in place and removed by hand; in closed-face methods the same obstacle stops the drive.

On the other hand, once the pipe has set off it cannot be steered. Line and grade are fixed by the pipe's initial positioning in the shaft; deviation that accumulates as the drive advances cannot be recovered. Pipe pushing is therefore suited to short crossings and lines with relatively generous grade tolerance. As distance grows, so does friction on the outside of the pipe; where necessary, lubricating bentonite is injected around the pipe to keep the jacking force reasonable.

Because people work inside the pipe, this is also the most safety-sensitive of the three methods: confined-space rules, forced ventilation, continuous gas and oxygen monitoring, face support and an evacuation plan are integral parts of the work.

It is known that such work is sometimes carried out with people sent into the tunnel without any safety measures at all. These cost-driven(!) situations cannot be undone when something goes wrong and, sadly, end in fatalities. It is worth remembering that these practices carry very heavy legal sanctions, are utterly outdated and dangerous, and that no justification can come before human life.

The shaft is sized to take the machine and its equipment; where nothing prevents it, it is usually planned at twice the machine length, because the machine sits behind the pipe it will push underground. From the face backwards the order is pipe (with the augers inside), machine and thrust wall. As the pipe is driven, the flights carry the spoil back inside the pipe and discharge it into the pit at the front of the machine; the spoil is then removed from the shaft by conveyor. The cutting head ahead of the leading auger is the first tool to touch the ground; its cutters and picks break the soil and create the void for the pipe to advance.

The critical balance of the drive is set here: excavation rate and jacking rate must match. If the auger draws too much soil, a void is left around the pipe and appears at the surface as settlement in time. If it draws too little, the head labours, torque and jacking force rise and equipment is damaged. The main gauge the operator reads throughout the job is the ratio of the two.

The jacking force applied on each project varies with the diameter of the pipe and the nature of the ground. Where essential, friction-reducing lubricants can be applied around the pipe. With equipment of 840 tonnes jacking force, Deltek can install pipes of up to 72" (~1830 mm) diameter by auger boring.

Above the 2000 mm diameter limit auger boring starts to lose its advantage. The jacks continue their job, but the excavation is done by machinery as in a classic tunnel drive, saving time and cost.

Unguided auger boring

One of the oldest systems known, and exactly the arrangement described above. Line is fixed by the machine's initial set-up in the shaft; deviation accumulated as the drive advances cannot be corrected. Because deviation shrinks as distance shortens, the method is readily used on pressure lines (potable water, natural gas, power and telecom casings) where grade accuracy is not critical.

Guided auger boring

In guided pipe jacking the pipe is driven into the ground along a trajectory whose coordinates were fixed in advance. A pilot bore is drilled first; the cutting head, augers and machine are then connected to this pilot so that the drive cannot wander off line. Guided pipe jacking is generally used on gravity wastewater and sewer lines, where it is not enough to bring the pipe to the right point — the gradient of the line must also be held to the millimetre.

When is the ground suitable?

Auger boring works best in self-supporting stiff clay, preferably above the water table. In flowing sand and ground below the water table the auger draws water along with the soil and creates voids. Bouldery and gravelly ground is a separate problem: the cutting head can neither break nor swallow a boulder, and with no access to the face the obstacle can stop the drive. This is why on auger boring projects the geotechnical survey comes before the choice of equipment.

Line is read continuously by a laser set up in the shaft and projected onto a target inside the machine; grade and roll are monitored by inclinometers on board. The shield is articulated and steering jacks inside it correct the course as it advances. Seeing deviation immediately and correcting it immediately is the feature that makes microtunnelling the preferred choice for gravity lines.

The jacking force comes from the main jacking frame in the shaft. As the line lengthens, friction on the outside of the pipe accumulates; two things balance it: continuous injection of bentonite around the pipe and, on long drives, intermediate jacking stations placed along the line. The intermediate stations split the load into parts, making lengths possible that a single frame could never push.

This is also where the cost comes from: a separation plant or foam system, a crane, power, larger shafts and a specialist crew. Microtunnelling is not the right tool for a short crossing; it makes sense where distance, accuracy, ground or water level rule out the other two methods.

Microtunnelling machines can be divided into two types by construction.

Slurry microtunnelling machines

These machines pump a bentonite-and-water suspension to the cutter head to stabilise the excavation face and mix it with the excavated soil. A closed-loop pipe system then carries the slurry–soil mixture to a separation plant on the surface, where the spoil is removed and the fluid recycled.

Balance at the face is achieved entirely by slurry pressure: the flow and pressure in the feed and return lines are adjusted to create a counter-pressure against the earth and water pressure. The separation plant consists of screens, cyclones and, where needed, a centrifuge; it removes the coarse fraction and recovers the bentonite, reducing both waste and water consumption.

This type excels in sand, gravel and ground below the water table — the closed-loop system takes water out of the equation. The price is the space needed on site for the separation plant, a water supply and extra power.

Earth pressure balance (EPB) machines

These machines balance earth and groundwater pressure using the excavated soil itself, conditioned with foaming agents or polymers inside a pressurised chamber, and generally remove the spoil by screw conveyor rather than by hydraulic slurry pumping.

In other words the support material does not come from outside; it is the excavated ground itself. Turned into a plastic, impermeable paste with foam and polymer additives, the soil is held under pressure in the cutter chamber; the screw conveyor takes this paste out in a controlled way while maintaining the pressure difference between the chamber and the shaft. The discharge rate is matched to the advance rate to keep the face pressure constant.

EPB is efficient in clay, silt and mixed ground, and because it needs no separation plant on site, its set-up is lighter than the slurry system. On the other hand its success depends entirely on getting the soil conditioning right: with the wrong additive ratio the paste either flows or clogs.

Which method for which project?

Three things decide: the type of ground, the groundwater level, and the length of the crossing together with its grade tolerance. Above the water table, in self-supporting ground and on a short crossing, auger boring is the fastest and most economical solution. Where obstacles requiring manual intervention at the face are expected and the diameter is large enough, pipe pushing makes sense. Below the water table, in flowing ground, over long distances or at gravity-line accuracy, microtunnelling comes into play. If you are not sure which group your project falls into, contact us with your geotechnical survey and let us determine the right method together.